fix: include non-default BLEU n-gram order in signatures

[leeshengis]

Fixes #251

Summary

Include ngram:N in BLEU reproducibility signatures when max_ngram_order differs from the default of 4, using n:N for short signatures. Default signatures remain unchanged, as discussed in the issue.

This enhancement distinguishes configurations that can produce different BLEU scores. It does not change score computation.

• Extend existing maximum-order tests to check full and short signatures for orders 1–6 while retaining the score assertions
• Check that the implicit default signature remains unchanged
• Document the optional signature field

Validation

• Regression tests before implementation: 10 non-default signature failures; 4 default-signature cases passed
• BLEU tests: 35 passed
• python -m pytest --ignore=test/test_dataset.py: 194 passed (including API dataset access tests)
• mypy sacrebleu scripts test: passed
• ruff check .: passed
• python -m build --no-isolation: passed
• Local CLI smoke checks: default long and short text signatures unchanged
• Focused CLI Japanese/Korean MeCab checks: 2 passed
• Full pytest attempted but did not finish within the local time limit while fetching test datasets; no full-suite pass is claimed
• Full CLI suite attempted; checks progressed through BLEU/chrF comparisons into TER before the local time limit, so no full CLI-suite pass is claimed

AI assistance was used to implement and test this change.